This application is a continuation-in-part of application Ser. No. 220,133, filed July 18, 1988 now abandoned.
The present invention relates to packaging films. In particular, the present invention relates to biaxially stretched films made of copolymers of polyethylene.
Polyethylene is the name for a polymer whose basic structure is characterized by the chain (CH.sub.2 CH.sub.2).sub.n. Polyethylene homopolymer is generally described as being a solid which has a partially amorphous phase and partially crystalline phase with a density of between 0.915 to 0.970 g/cm.sup.3. The relative crystallinity of polyethylene is known to affect its physical properties. The amorphous phase imparts flexibility and high impact strength while the crystalline phase imparts a high softening temperature and rigidity.
Unsubstituted polyethylene is generally referred to as high density homopolymer and has a crystallinity of 70 to 90 percent with a density between about 0.96 to 0.97 g/cm.sup.3. Most commercially utilized polyethylenes are not unsubstituted homopolymer but instead have C.sub.2 -C.sub.8 alkyl groups attached to the basic chain. These substituted polyethylenes are also known as branched chain polyethylenes. Also, commercially available polyethylenes frequently include other substituent groups produced by copolymerization. Branching with alkyl groups generally reduces crystallinity, density and melting point. The density of polyethylene is recognized as being closely connected to the crystallinity. The physical properties of commercially available polyethylenes are also affected by average molecular weight and molecular weight distribution, branching length and type of substituents.
People skilled in the art generally refer to several broad categories of polymers and copolymers as "polyethylene". Placement of a particular polymer into one of these categories of "polyethylene" is frequently based upon the density of the "polyethylene" and often by additional reference to the process by which it was made since the process often determines the degree of branching, crystallinity and density. In general, the nomenclature used is nonspecific to a compound but refers instead to a range of compositions. This range often includes both homopolymers and copolymers.
For example, "high density" polyethylene (HDPE) is ordinarily used in the art to refer to both (a) homopolymers of densities between about 0.960 to 0.970 g/cm.sup.3 and (b) copolymers of ethylene and an alpha-olefin (usually 1-butene or 1-hexene) which have densities between 0.940 and 0.958 g/cm.sup.3. HDPE includes polymers made with Ziegler or Phillips type catalysts and is also said to include high molecular weight "polyethylenes". In contrast to HDPE, whose polymer chain has some branching, are "ultrahigh molecular weight polyethylenes" which are essentially unbranched specialty polymers having a much higher molecular weight than the high molecular weight HDPE.
Hereinafter, the term "polyethylene" will be used (unless indicated otherwise) to refer to ethylene homopolymers as well as copolymers of ethylene with alpha-olefins and the term will be used without regard to the presence or absence of substituent branch groups.
Another broad grouping of polyethylene is "high pressure, low density polyethylene" (LDPE). The polyethylene industry began in the 1930's as a result of the discovery of a commercial process for producing LDPE by Imperial Chemical Industries, Ltd. researchers. LDPE is used to denominate branched homopolymers having densities between 0.915 and 0.930 g/cm.sup.3 as well as copolymers containing polar groups resulting from copolymerization e.g. with vinyl acetate or ethyl acrylate. LDPEs typically contain long branches off the main chain (often termed "backbone") with alkyl substituents of 2 to 8 carbon atoms.
In the 1970's a new grouping of polyethylene was commercialized--Linear Low Density Polyethylene (LLDPE). Only copolymers of ethylene with alpha-olefins are in this group, LLDPEs are presently recognized by those skilled in the art as having densities from 0.915 to 0.940 g/cm.sup.3 The alpha-olefin utilized is usually 1-butene, 1-hexene, or 1-octene and Ziegler-type catalysts are usually employed (although Phillips catalysts are also used to produce LLDPE having densities at the higher end of the range).
In the 1980's yet another grouping of polyethylene has come into prominence--Very Low Density Polyethylene (VLDPE) which is also called "ultra low density polyethylene" (ULDPE). This grouping like LLDPEs comprise only copolymers of ethylene with alpha-olefins, usually 1-butene, 1-hexene or 1-octene. However, VLDPEs have lower densities than LLDPEs. The densities of VLDPEs are recognized by those skilled in the art to range between 0.860 and 0.915 g/cm.sup.3. A process for making VLDPEs is described in European Patent Document publication number 120,503 whose text and drawing are hereby incorporated by reference into the present document.
Various types of polyethylene resins have long been used to produce films having different properties. These polyethylenes have been used alone, in blends and with copolymers in both monolayer and multilayer films for packaging applications for such food products as poultry, fresh red meat and processed meat. In the food industry greater use of centralized processing of foods in conjunction with increased handling and long distance transportation have increased the demand for packaging films having superior properties.
It is generally known that selection of films for packaging food products include consideration of one or more criteria such as puncture resistance, shrinkability, shrink force, cost, sealability, stiffness, strength, printability, durability, barrier properties, machinability, optical properties such as haze and gloss, flex-crack resistance and government approval for contact with food.
For example, several film materials containing polyethylene have been either used or proposed for packaging frozen poultry. In general, commercial poultry packaging operations require bags made from materials able to withstand the following typical process and transfer
1. Inserting a bird into a bag fabricated from a shrinkable film; PA1 2. Evacuating the bag; PA1 3. Clamping or otherwise sealing the neck of the bag; PA1 4. Transporting the bird (e.g. by a conveyor belt) to a shrink tunnel; PA1 5. Shrinking the bag tightly around the bird by exposing the bag to a temperature of about 90-95.degree. C. for up to about six to eight seconds; PA1 6. Quick freezing and storage of the packaged bird at temperatures as low as -40.degree. C.; and PA1 7. Transporting the packaged bird from the commercial packer to the ultimate user. PA1 a) A shrinkage value that yields a reduction in the area of the film at a temperature from 90-95.degree. C. that is sufficient to conform the film to the irregular shape of the bird; PA1 b) a shrink force at a temperature of 90-95.degree. C. is required that is sufficient to pull the wings of the bird in tightly toward the body with sufficient residual shrink force to maintain a tight wrap around the bird; and PA1 c) a puncture resistance sufficient to withstand the packaging operation itself as well as subsequent transport of the packaged bird.
A film useful for frozen poultry packaging will include among its desirable properties the following:
All the above properties should be provided in a film at a minimum of cost.
Several polyolefin films have previously been proposed for use as poultry bags.
U.S. Pat. No. 3,555,604 (Pahlke) discloses that low density polyethylene may be biaxially oriented to produce a film which is useful for packaging food stuffs such as turkey.
U.S. Pat. No. 3,365,520 (Foster et al) discloses a bioriented film of a blend of an ionomer and an ethylene homopolymer or copolymer.
Multilayer bioriented films have also been proposed such as those disclosed in U.S. Pat. No. 3,900,635 (Funderburk, Jr. et al) having a first layer of ethylene homopolymer or copolymer and a second layer of a blend of an ionomer and an ethylene homopolymer or copolymer. The multilayer film of Funderburk, Jr. et al is said to have a shrinkage of at least 25% in the transverse direction at 90-95.degree. C.
Yet another biaxially oriented film for poultry bags is described in U.S. Pat. No. 4,547,413 (Lustig et al) wherein a mixture of ethylene-vinyl acetate copolymer and either low pressure high density polyethylene or low pressure low density polyethylene is suggested.
Also, various blends of different polyethylene resins have been reported. For example, blends of LLDPE with LLDPE or LDPE have been reported in the article by Utracki et al, "Linear Low Density Polyethylene and Their Blends: Part 4 Shear Flow of LLDPE Blends with LLDPE and LDPE", Polymer Engineering and Science. Vol. 27, No. 20, pp 1512-1522 (mid-November, 1987). In its introduction, the above article states that ... "at least 60% of LLDPE is sold in blends with polyolefins or EVA (ethylene-vinyl acetate copolymers) (cite omitted). Amelioration of properties (e.g., puncture resistance), lowering of material cost or improvement of processability are the main reasons". The article goes on to discuss data relating to blends of a LLDPE made from a copolymer of polyethylene with 1-butene with (a) a LLDPE made from a copolymer of polyethylene and 1-hexene, and (b) a LDPE.
VLDPE has been suggested for use as a blending resin with HDPE, polypropylene, ethylene-vinyl acetate copolymer and ethylene-propylene copolymer (see e.g. Plastics Technology. p.13 October, 1984). Also, European Patent Document 236,099 discloses a shrinkable multilayer film having outer layers which can comprise a blend of LLDPE and VLDPE.
U.S. Pat. No. 4,640,856 (Ferguson et al) discloses heat shrinkable multilayer films containing VLDPE which are useful in packaging meat, poultry and dairy products. Ferguson, et al in describing their thermoplastic polymeric layer also states that "in certain applications blends of VLDPE, LLDPE and/or EVA may be used to achieve desired properties".
U.S. Pat. No. 4,671,987 (Knott, II et al) discloses a printable composite stretch wrap multilayer film which may comprise a layer of VLDPE. Knott, II et al state that "The VLDPE can be blended with from 0 to about 50 wt.% of another ethylene polymer such as polyethylene homopolymer, ethylene- vinyl acetate copolymer, linear low density polyethylene, and mixtures of these compounds".
U.S. Pat. No. 4,720,427 (Clausen et al) discloses an oriented multi-layer heat sealable film, Clausen et al state that this film may contain a layer comprising a blend of VLDPE with a "polyolefin". Blends of VLDPE with polypropylene are particularly discussed.
U.S. Pat. No. 4,726,997 (Mueller et al) discusses blends of VLDPE with modified ethylene propylene copolymer and/or sodium carbonate.
None of the foregoing patents have disclosed specific blends of very low density polyethylenes having surprisingly good physical properties. Also, presently known films used as poultry bags continue to suffer from insufficient puncture resistance, shrinkability and/or shrink force.
Punctured and leaking bags are still very much a problem in poultry packaging. Films made of 1-butene very low density polyethylene alone have undesirably low hot water puncture resistance, but 1-butene VLDPE does have good shrinkability. Films made of 1-hexene VLDPE have excellent hot water puncture properties but poor shrinkability. Furthermore, the shrink force of both films in the machine direction is undesirably low at both elevated temperature and residually at room temperature. Also, it would be highly desirable for poultry bags to have improved dynamic puncture resistance with good shrinkability.